Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium on a disc. Modern disc drives comprise one or more rigid discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers ("heads") mounted to a radial actuator for movement of the heads relative to the discs. Each of the concentric tracks is generally divided into a plurality of separately addressable data sectors. The read/write transducer, e.g. a magnetoresistive read/write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment. Critical to both of these operations is the accurate locating of the head over the center of the desired track.
The heads are mounted via flexures at the ends of a plurality of actuator arms that project radially outward from the actuator body. The actuator body pivots about a shaft mounted to the disc drive housing at a position closely adjacent the outer extreme of the discs. The pivot shaft is parallel with the axis of rotation of the spindle motor and the discs, so that the heads move in a plane parallel with the surfaces of the discs.
Typically, such radial actuators employ a voice coil motor to position the heads with respect to the disc surfaces. The actuator voice coil motor includes a coil mounted on the side of the actuator body opposite the head arms so as to be immersed in the magnetic field of a magnetic circuit comprising one or more permanent magnets and magnetically permeable pole pieces. When controlled direct current (DC) is passed through the coil, an electromagnetic field is set up which interacts with the magnetic field of the magnetic circuit to cause the coil to move in accordance with the well-known Lorentz relationship. As the coil moves, the actuator body pivots about the pivot shaft and the heads move across the disc surfaces. The actuator thus allows the head to move back and forth in an arcuate fashion between an inner radius and an outer radius of the discs.
The actuator arm is driven by a control signal fed to the voice coil motor (VCM) at the rear end of the actuator arm. A servo system is used to sense the position of the actuator and control the movement of the head above the disc using servo signals read from a disc surface in the disc drive. The servo system relies on servo information stored on the disc. The signals from this information generally indicate the present position of the head with respect to the disc, i.e., the current track position. The servo system uses the sensed information to maintain head position or determine how to optimally move the head to a new position centered above a desired track. The servo system then delivers a control signal to the VCM to rotate the actuator to position the head over a desired new track or maintain the position over the desired current track.
Servo information is typically stored in a disc drive in one of two ways: sectored servo and dedicated servo. In a sectored servo system, servo information is interspersed with user data on the disc surface. The servo information is stored in arcuate segments on each track of the disc surface. These segments are interspersed around the track between stored user data on the track. As the disc rotates beneath the head, the head periodically samples the servo sectors to obtain the servo information for the servo system. In a dedicated servo system, the servo information is stored on a separate dedicated disc surface which contains no user data. In this system, servo information is constantly available to the servo control system.
In a sectored servo system, the servo information that is recorded on a servo segment consists typically of a burst pattern. Table 1 below sets forth the fields of a typical set of servo segment dibits that make up the servo burst pattern in a servo segment. Table 2 breaks down, as an example, the prefix field dibits.
TABLE 1 AGC Gray A B C. D Description Preamble AM Prefix Code Pad Burst Pad Burst Pad Burst Pad Burst Pad Length in Dibits 20 8 9 16 2 8 2 8 2 8 2 8 2
TABLE 2 Bit 8 7 6 5 4 3 2 1 0 Value 1 1 X 1 1 X 1 0 1 Descrip- AM Index Guard Zap Guard tion Synch Bit Bit
The total number of dibits in a conventional servo field pattern is typically 95. This is made up of the automatic gain control block of 20 dibits, followed by an 8 dibit Address Mark and a 9-digit Prefix field. Currently the track number is identified by 16 bits of Gray code. The gray code is followed by four position bursts (A, B, C, D), each 8 bits long, separated by two dibit pads. Servo information represents system overhead that reduces the amount of user data that can be accommodated on the disc drive. It is thus desirable to minimize the disc surface real estate required for storage of servo information since storage capacity of as much user data as possible on the disc is generally desired.